This application is based upon Japanese Patent Application No. 2000-187698 filed on Jun. 22, 2000, the contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a load drive circuit having a charge pump circuit for raising a voltage, and a switch circuit for opening and closing a power supply path to a load by using this raised voltage.
2. Related Art
FIG. 10 shows the conventional construction of a load drive circuit for driving a load (a relay, a light, etc.) mounted to an automobile. The load drive circuit 1 is constructed by a MOSFET 4 for controlling a conducting state of the load 2, a drive circuit 6 for driving this MOSFET 4, a power circuit 8, a voltage raising (charge pump) circuit 5 for raising and outputting an inputted voltage, and a low voltage detecting circuit 7. The charge pump circuit 5, the drive circuit 6 and the low voltage detecting circuit 7 are activated by receiving the supply of a controlled power voltage from the power circuit 8 with a battery voltage Vb as an input.
The MOSFET 4 is interposed on a power line 3 (a power supply path) for connecting an unillustrated battery and the load 2. The charge pump circuit 5 inputs the battery voltage Vb on the power line 3, and outputs a raised voltage Vc. The drive circuit 6 receives the supply of the raised voltage Vc from this charge pump circuit 5, and operates the MOSFET 4 in accordance with a drive command signal Sd. The low voltage detecting circuit 7 is arranged to execute a function (a so-called power on reset function) for turning-off an output when the battery voltage Vb is a low voltage. For example, when the battery voltage Vb is lower than a predetermined threshold value Vd, the power circuit 8 cannot generate a stable control power voltage, so that the load drive circuit 1 is not operated normally. Therefore, the low voltage detecting circuit 7 detects the battery voltage Vb by a voltage dividing circuit arranged within this low voltage detecting circuit 7. When this detected voltage is lower than a reference voltage Vr corresponding to the threshold value Vd (in the case of a low voltage state), the low voltage detecting circuit 7 outputs a low voltage detecting signal Se at a high voltage level to the drive circuit 6. When the drive circuit 6 receives this low voltage detecting signal Se at a high voltage level, the drive circuit 6 turns off the MOSFET 4.
FIG. 11 shows a voltage waveform of each portion and a state of the low voltage detecting signal Se when the battery voltage Vb becomes lower than the threshold value Vd and is then raised in a providing state of the drive command signal Sd for instructing turning-on of the MOSFET 4. Here, the voltage waveform of each portion is set as follows.
Waveform A (thick solid line): battery voltage Vb
Waveform B (thick solid line): detecting voltage Va obtained by dividing battery voltage Vb
Waveform C (thin solid line): output voltage Vo
In FIG. 11, since the battery voltage Vb (detecting voltage Va) is equal to or greater than the threshold value Vd (reference voltage Vr) before a time t1, the low voltage detecting circuit 7 outputs the low voltage detecting signal Se at a low voltage level, and the drive circuit 6 continuously turns on the MOSFET 4. In this case, the output voltage Vo (a source voltage of the MOSFET 4) is lower than the battery voltage Vb by the voltage between a gate and a source of the MOSFET 4, and an electric current flows from the battery to the load 2 through the power line 3 and the MOSFET 4.
When the time t1 has shortly passed and the battery voltage Vb (detecting voltage Va) is lower than the threshold value Vd (reference voltage Vr), the low voltage detecting circuit 7 sets the low voltage detecting signal Se from the low voltage level to a high voltage level so that the drive circuit 6 turns off the MOSFET 4. When the MOSFET 4 is turned off, the electric current flowing to the load 2 is interrupted, and the output voltage Vo is reduced to 0 V (earth electric potential).
At this time, the electric current flowing through the power line 3 from the battery is suddenly reduced. Therefore, for example, the battery voltage Vb detected in the load drive circuit 1 is temporarily jumped up by an inductance component on the power line 3, and exceeds the threshold value Vd. As a result, the low voltage detecting signal Se becomes the low voltage level, and the drive circuit 6 again turns on the MOSFET 4. When the electric current flowing through the power line 3 is suddenly increased by turning on the MOSFET 4, the battery voltage Vb becomes lower than the threshold value Vd, and the low voltage detecting signal Se becomes the high voltage level. Accordingly, the drive circuit 6 again turns off the MOSFET 4.
The MOSFET 4 finally lapses into an oscillating state repeating turning-on and turning-off by the jumping-up of the battery voltage Vb caused in the turning-off of the MOSFET 4 while the battery voltage Vb becomes higher than the threshold value Vd (period T1). This phenomenon is similarly caused when the battery voltage exceeds the threshold value Vd in a raising process of the battery voltage Vb (a period T2 from a time t2).
When the MOSFET 4 falls into the oscillating state, switching loss of the MOSFET 4 is increased and surge voltage is generated. Therefore, it is necessary to adopt an element of large allowable loss and an element of high withstand voltage so that cost is increased and parts are large-sized. Further, in a period in which the MOSFET 4 is in the oscillating state, there may be a problem that chattering is generated in a relay switch when the load 2 is a relay coil. When the load 2 is a light, there may be also a problem that the light is turned on and off.
For example, it is considered as a means for preventing the generation of such an oscillating state that hysteresis characteristics are added to a comparison circuit of the detecting voltage Va and the reference voltage Vr in the low voltage detecting circuit 7. In this case, since the above jumping-up voltage width is considerably large, a hysteresis width constructed by lower and upper threshold values is correspondingly set to be wide.
It is necessary to set the lower threshold value to the threshold value Vd to reliably turn off the MOSFET 4 in a period in which the battery voltage Vb is lower than the predetermined threshold value Vd as a function for interrupting an output when the voltage is low. When the battery voltage Vb lies between the lower and upper threshold values (in a hysteresis voltage area), the MOSFET 4 attains the turning-off state at a raising state of the battery voltage Vb, and attains the turning-on state at a lowering state of the battery voltage Vb. Accordingly, it has became apparent that a minimum operating voltage of the load drive circuit 1 capable of performing turning-on and turning-off operations of the MOSFET 4 in accordance with the drive command signal Sd is increased.
In view of the above situations, an object of the present invention is to provide a reliable load drive circuit having a low voltage time output off function and preferably operated even when the load drive circuit is arranged in a power supply path in which a power voltage is temporarily changed by a sudden change in a load electric current.
In accordance with the present application invention in a first aspect, a load drive circuit is characterized in that the load drive circuit comprises:
a charge pump circuit for inputting a power voltage and outputting a raised voltage;
a switch circuit arranged in a power supply path for connecting a power source and a load, and setting said power supply path to an opening state or a closing state in accordance with a drive command signal in a supply state of an output voltage of said charge pump circuit; and
a low voltage detecting circuit for detecting the output voltage of said charge pump circuit as a low voltage state when this output voltage of said charge pump circuit is lower than a predetermined judging level;
wherein said switch circuit is constructed such that said switch circuit sets said power supply path to the opening state irrespective of said drive command signal in a period in which said low voltage detecting circuit detects said low voltage state.
Accordingly, the switch circuit is operated such that the power supply path is set to the opening or closing state in accordance with the drive command signal in a state in which the output voltage of the charge pump circuit is supplied. The charge pump circuit generally has a construction in which a charging circuit constructed by a diode and a capacitor for preventing the back flow of electric charges is in a cascade connection by a required stage number. Further, constant voltage control is not performed.
In such a construction, the charge pump circuit outputs the raising voltage according (e.g., proportional) to the inputted power voltage. In the charge pump circuit, the electric charges are sequentially moved from the charging circuit located on an input side to the charging circuit located on an output side, so that a delay occurs until a variation of the inputted power voltage appears in the output. Further, the voltage variation appearing in the output of the charge pump circuit is small in comparison with the variation of the inputted power voltage by an action of the capacitor as a low pass filter. Further, the diode for preventing the back flow of electric charges exists in the charging circuit. Therefore, temporary reduction in the inputted power voltage does not easily appear in the output.
As a result, the low voltage detecting circuit can indirectly detect the low voltage state of the power voltage by detecting the low voltage state with respect to the output voltage of the charge pump circuit. The switch circuit sets the power supply path to the opening state irrespective of the drive command signal in a detecting period of this low voltage state. Accordingly, it is possible to prevent the generations of an unstable operation of the load drive circuit and an error in the operation caused by a reduction in the power voltage so that the load drive circuit and the load can be protected (low voltage time output off function).
An inductance component generally exists in the power supply path. Accordingly, when the switch circuit energizes and de-energizes the load by opening and closing the power supply path, a phenomenon of temporarily varying the voltage of the power supply path is caused. In contrast to this, the present means does not directly detect the voltage of the power supply path, but indirectly detects this voltage through the output voltage of the charge pump circuit. Accordingly, temporary voltage variation generated in turning the load on and off is not easily detected by the above characteristics of the charge pump circuit. As a result, when the power voltage is reduced and is lower than a threshold value, it is possible to prevent the generation of an oscillating state alternately repeating the opening and closing operations of the power supply path due to the switch circuit without separately arranging a filter circuit.
Further, in accordance with the present means, it is not necessary to add hysteresis to prevent the temporary voltage variation caused by the opening and closing operations of the power supply path so that a minimum operation voltage can be set to be low.
These and other objects, features and characteristics of the present invention will be appreciated from a study of the following detailed description, the appended claims, and drawings, all of which form parts of this application. In the drawings, same portions or corresponding portions are put the same numerals each other to eliminate redundant explanation.